Wind tunnel and numerical study of a small vertical axis wind turbine

A data–informed approach is presented with the objective of quantifying errors and uncertainties in the functional forms of turbulence closure models. The approach creates modeling information from higher-fidelity simulations and experimental data. Specifically, a Bayesian formalism is adopted to infer discrepancies in the source terms of transport equations. A key enabling idea is the transformation of the functional inversion procedure (which is inherently infinite-dimensional) into a finite-dimensional problem in which the distribution of the unknown function is estimated at discrete mesh locations in the computational domain. This allows for the use of an efficient adjoint-driven inversion procedure. The output of the inversion is a full-field of discrepancy that provides hitherto inaccessible modeling information. The utility of the approach is demonstrated by applying it to a number of problems including channel flow, shock-boundary layer interactions, and flows with curvature and separation. In all these cases, the posterior model correlates well with the data. Furthermore, it is shown that even if limited data (such as surface pressures) are used, the accuracy of the inferred solution is improved over the entire computational domain. The results suggest that, by directly addressing the connection between physical data and model discrepancies, the field inversion approach materially enhances the value of computational and experimental data for model improvement. The resulting information can be used by the modeler as a guiding tool to design more accurate model forms, or serve as input to machine learning algorithms to directly replace deficient modeling terms.

Using field inversion to quantify functional errors in turbulence closures

Several 2D and 3D Navier–Stokes computations performed with the compressible elsA solver, developed at ONERA, on the wind turbine tested by NREL in the NASA Ames large wind tunnel are presented. After a brief description of the tools and methods used, the predictions of the S809 aerofoil performance are first discussed. Full 3D computations are then presented, analysed and compared with the experimental results. The successes and failures of the computations are highlighted and explained. The analysis focuses in particular on the 3D and unsteady effects. Copyright © 2004 John Wiley & Sons, Ltd.

3D Navier–Stokes computations of a stall‐regulated wind turbine

The unsteady flow past a NACA 0012 airfoil that is undertaking a constant-rate pitching up motion is investigated experimentally by the PIDV technique in a water towing tank. The Reynolds number is 5000, based upon the airfoil's chord and the free-stream velocity. The airfoil is pitching impulsively from 0 to 30 deg. with a dimensionless pitch rate alpha of 0.131. Instantaneous velocity and associated vorticity data have been acquired over the entire flow field. The primary vortex dominates the flow behavior after it separates from the leading edge of the airfoil. Complete stall emerges after this vortex detaches from the airfoil and triggers the shedding of a counter-rotating vortex near the trailing edge. A parallel computational study using the discrete vortex, random walk approximation has also been conducted. In general, the computational results agree very well with the experiment.

Unsteady flow past an airfoil pitching at a constant rate

A parafoil controlled by parafoil brake dee ection offers a lightweight and space-efe cient control mechanism for autonomous placement of air-dropped payloads to specie ed ground coordinates. The work reported here investigates control issues for a parafoil and payload system with left and right parafoil brakes used as the control mechanism.Itisshownthatparafoilandpayloadsystemscanexhibittwobasicmodesoflateralcontrol,namely,roll and skidsteering.Thesetwo modesoflateralsteeringgeneratelateralresponseinoppositedirections.Forexample, a roll steer cone guration turns left when the right parafoil brake is activated, whereas a skid steer cone guration turnsright under the samecontrol input. In transition between roll and skid lateral steering, the lateral responseis zero, and the system becomes uncontrollable. Angle of incidence, canopy curvature of the parafoil, and magnitude of brake dee ections are important design parameters for a controllable parafoil and payload system and greatly effect control response, including whether the basic lateral control mode is roll or skid steering. It is shown how the steering mode switches when fundamental design parameters are altered and as the magnitude of the brake dee ection increases. The mode of directional control transitions toward roll steering as the canopy curvature decreases or the angle of incidence becomes more negative. The mode of directional control transitions away from the roll steering mode as the magnitude of the brake dee ection increases, and for “ large” brake dee ections most parafoils will always skid steer.

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Aspects of Control for a Parafoil and Payload System

Steady-state, two-dimensional CFD calculations were made for the S809 laminar-flow, wind-turbine airfoil using the commercial code CFD-ACE. Comparisons of the computed pressure and aerodynamic coefficients were made with wind tunnel data from the Delft University 1.8 m x 1.25 m low-turbulence wind tunnel. This work highlights two areas in CFD that require further investigation and development in order to enable accurate numerical simulations of flow about current generation wind-turbine airfoils: transition prediction and turbulence modeling. The results show that the laminar-to-turbulent transition point must be modeled correctly to get accurate simulations for attached flow. Calculations also show that the standard turbulence model used in most commercial CFD codes, the k-{epsilon} model, is not appropriate at angles of attack with flow separation.

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CFD calculations of S809 aerodynamic characteristics

The incompressible three-dimensional Navier–Stokes equations are solved numerically for a fluid-filled cylindrical cannister that is spinning and nutating. The motion of the cannister is characteristic of that experienced by spin-stabilized artillery projectiles. Equations for the internal fluid motion are derived in a non-inertial aeroballistic coordinate system. Steady-state numerical solutions are obtained by an iterative finite-difference procedure. Flow fields and liquid induced moments have been calculated for viscosities in the range of 0.9 × 104−1 × 109 cSt. The nature of the three-dimensional fluid motion inside the cylinder is discussed, and the moments generated by the fluid are explained. The calculated moments generally agree with experimental measurements.

Fluid motion inside a spinning nutating cylinder

An experiment was conducted in Arnold Engineering Development Center's 16-ft transonic wind tunnel to measure the dependency of vortex-induced counter torque upon J (the ratio of spin motor jet dynamic pres- sure to freestream dynamic pressure), Mach number, Reynolds number, angle of attack and roll orientation, spin motor nozzle configuration, and fin cant angle. Counter torque data and Laser Vapor Screen images confirm that J is the dominant parameter for correlating counter torque produced by a given vehicle configura- tion, flight condition, angle of attack and roll orienta- tion. At M = 0.8 (with no shock waves in the flow), we observed a monotonic variation of the counter torque coefficient CCT with J that is independent of Reynolds number but dependent on angle of attack and the orientation of the fins with respect to the spin motor nozzle azimuthal location. At M = 0.95 and 1.1, meas- ured values of CCT were strongly influenced by changes in Reynolds number, suggesting that shock-boundary layer interaction may be present. Nomenclature A model cross-sectional area CCT counter torque coefficient, Eq. 4 CP center of pressure CT counter torque due to vortex-fin interaction D model diameter J plume-freestream interaction parameter,

Experiments and computations of roll torque induced by vortex-fin interaction

The primary purpose of this paper is to provide an evaluation of the diffusion velocity concept with regard to its ability to predict the diffusion of vorticity near moving walls as well as in unbounded two-dimensional space. We also briefly outline a "bi-zonal" wall model which utilizes the diffusion velocity concept and is compatible with vortex methods. A primary goal of this model is to minimize the number of vortices generated on the surface at each time step while achieving good resolution of the vorticity field near the wall. We have generated results for an initially square patch of vorticity in an unbounded domain, an impulsively started flat plate, an oscillating flat plate, uniform flow over a semi-infinite flat plate, and flow over a wedge. The simulations are found to give excellent results when compared to classical solutions of these problems.

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Viscous diffusion of vorticity using the diffusion velocity concept

In this paper the authors present the current status of an unsteady 3D parachute simulation code which is being developed at Sandia National Laboratories under the Department of Energy's Accelerated Strategic Computing Initiative (ASCI). The Vortex Inflation PARachute code (VIPAR) which embodies this effort will eventually be able to perform complete numerical simulations of ribbon parachute deployment, inflation, and steady descent. At the present time they have a working serial version of the uncoupled fluids code which can simulate unsteady 3D incompressible flows around bluff bodies made up of triangular membrane elements. A parallel version of the code has just been completed which will allow one to compute flows over complex geometries utilizing several thousand processors on one of the new DOE teraFLOP computers.

/pdf/on-the-development-of-a-gridless-inflation-code-for-1hh59xr4d6.pdf

Walter P. Wolfe

Papers

CFD calculations of S809 aerodynamic characteristics

Fluid motion inside a spinning nutating cylinder

Experiments and computations of roll torque induced by vortex-fin interaction

Viscous diffusion of vorticity using the diffusion velocity concept

On the Development of a Gridless Inflation Code for Parachute Simulations